Developmental processes can be viewed as a delicate balance between continued proliferation and a switch to termin. differentiation. This transition is often controlled by transcriptional regulators, which if damaged can cause disease. To better understand such developmental processes and potentially be able to prevent oncogenesis, the investigators are studying the spatial expression and function of an important class of developmental regulators in vertebrates, the Hox genes. Control of spatial expression is poorly understood, but it is clearly a crucial mechanism by which proper determination of positional cues by the HOX proteins occurs during development. The investigators' overarching hypothesis is that HOX proteins transduce regional information through being expressed in highly specific regional subsets of their total transcriptional pattern. To investigate this question they have identified elements controlling expression in the brachial/cervical region (BSCE) in mouse. Mesodermal structures specifically affected in Hoxa5 mutant mice develop from this spatial region; therefore such elements are central to Hox5 functional expression. Their specific aims to test this hypothesis and to investigate Hoxa5 expression and function are to: 1. Determine the in vivo function of the BSCE sequence through the analysis of a mouse line carrying a targeted mutation of the sequence. 2. Characterize the regulatory sequences and factors specifying Hoxa5 expression in the cervical region during embryogenesis. 3. Identify additional regulatory proteins involved in Hoxa5 gene regulation using the BSCE sequence and newly identified mesodermal elements. 4. Establish the structure and function of a novel putative Hoxa5 transcription factor, termed 0RF2, that recognizes specifically the BSCE sequence. This work will determine if Hoxa5 expression is required specifically in the brachial region through the BSCE, and will allow specific manipulations of Hoxa5 gene expression in vivo to address where Hoxa5 is required. They will also identify and characterize additional Hoxa5 regulators. Finally, they will study in more detail the novel orf gene and determine if it is a Hoxa5 regulator. This would be the first Hoxa5 regulator established. Completion of these aims will provide critical information required for elucidating the pathway controlling Hoxa5 expression, information useful to develop a detailed understanding of regional patterning during development.